1. Technical Field
The present invention relates to a method of forming an image without a cleaner and particularly to such a method which may be applied to a copying machine such as laser printers and photocopying machines using electrophotographic technology.
2. Background Art
Generally, when the above mentioned type of copying machine is used, image information is exposed onto a photosensitive drum to form an electrostatic latent image on an exposed portion and then toner is fed to the latent image to form a visible image (toner image). After that, the toner image is transferred to a sheet of paper by a transfer unit to obtain a copy.
The toner remaining on the photosensitive drum (some toner is not transferred to the paper) must be completely removed from the surface of the photosensitive drum. Otherwise, the quality of the next copy suffers considerably. Therefore, generally the photosensitive drum which has completed the image transferring step is cleaned prior to the next copying procedure. The cleaning is usually conducted by a cleaner provided at a lateral portion of the photosensitive drum. The cleaner has a blade which scrapes the remaining toner off the photosensitive drum. The scraped off toner is then guided into a collecting room by a guide sheet which extends below the blade.
Some copying machines employ a conventional image forming method which does not have a cleaner. In such a copying machine, a developing unit performs a developing process as well as a toner recovering process. Specifically, the toner remaining on the photosensitive drum is collected by the developing unit. After the transferring step, a memory removing member such as brush and electroconductive rubber roller is applied to the photosensitive drum to smudge and spread the toner remaining on the photosensitive drum even when image transferring has not taken place. This spreading step is referred to as "memory erasure step" in this particular specification. The remaining toner image left by the unused image transfer toner is erased in this way. Next, the photosensitive drum, used in the following image forming steps of charging and exposing, proceeds to the developing step.
FIG. 4 of the accompanying drawings shows the relative setting voltages of various components of a copying machine which employs a conventional image forming method without a cleaner. As seen in the illustration, the setting voltages have the following relation: Charging voltage on the photosensitive drum surface (V.sub.0)&lt;Bias voltage of developing unit (V.sub.1)&lt;Voltage of a portion exposed by the exposing unit (V.sub.2)&lt;Zero volt (0 V)&lt;Voltage of the photosensitive drum surface after the transfer and charge removal (V.sub.3)&lt;Voltage of memory removing member (V.sub.4).
The potential difference between V.sub.1 and V.sub.2 causes the development. The potential difference between V.sub.2 and V.sub.4 causes the transferring. The potential difference between V.sub.3 and V.sub.4 causes the memory removal. Since the setting voltage of the memory removing member is higher than the photosensitive drum surface voltage after the transfer and charge removal, the toner remaining on the photosensitive drum surface is lifted on the memory removing member. Then, some toner overflows from the memory removing member and returns onto the photosensitive drum surface. In this way, the remaining toner image left on the photoelectric drum by the unused image transfer toner is erased.
FIG. 3(c) of the accompanying drawings is a time chart showing a printing sequence according to a conventional image forming method which does not have a cleaner. In this time chart, since the photosensitive drum is rotated at a time t1, a charging device and a charge-removing lamp are turned on simultaneously with the motor. At a time t2, i.e., when the motor reaches a predetermined revolution speed, the developing unit and the memory removing member are turned on, respectively. At a time t3, the transfer unit is turned on so as to transfer the toner onto the sheet. Then, at a time t5, i.e., when the transferring step is completed, the transfer unit is turned off. At a time t7, the motor is turned off, and accordingly the revolution speed of the motor drops and the motor stops at a time t8. At the same time (t8), the electric charger, the developing unit, the charge-removing lamp and the memory removing member are turned off, respectively.
According to the conventional copying machine which depends on the above described image forming method without a cleaner, "jamming" occurs, and at the time that the image forming process is interrupted, there is a large amount of toner left on the photosensitive drum ready for image transfer. When jamming is eliminated and the image forming process resumes in this state, the toner holding capacity of the memory removing member is greatly exceeded and its performance is considerably deteriorated. Specifically, the remaining toner image left by the unused image transfer toner cannot be sufficiently removed and an undesired situation arises, i.e., during the next image forming process, an old toner image remains. In addition, the toner held in the memory removing member overflows from the memory removing member and stains peripheral elements and the sheet under the photosensitive drum etc.
Even if jamming does not occur, it is unavoidable that the memory performance will lower as the image forming process is repeated frequently. Therefore, the conventional without-cleaner image forming method actually needs a cleaning step for the memory removing member at an appropriate time.